Method and apparatus for spray coating of articles



May 1, 1956 J. w. JUVINALL METHOD AND APPARATUS FOR SPRAY comma OFARTICLES Filed NOV. 28, 1951 INVENTOR.

JAMES W. J UVINALL United States Patent METHOD AND APPARATUS FOR SPRAYCOATING OF ARTICLES James W. Juvinall, Brendonwood, Ind., assignor toRansburg Electro-Coafing Corp., a corporation of Indiana Thisapplication relates to the spray coating of articles and moreparticularly to the electrostatic deposition of liquid coating materialon articles.

When spaced articles are successively exposed to the spray issuing froma spraying device in an-electrostatic coating system where deposition ofthe coating material particles is primarily the result of electrostaticforces and where the particles are given an electric charge at the timethey leave the spraying device or shortly thereafter, the commercialpracticability of handling the articles on a rapidly moving conveyor hasheretofore sometimes resulted in the deposition of coating on thearticles in an uneven manner resulting in undersprayed or oversprayedareas.

In a majority of electrostatic coating systems a field of electrostaticforce is set up which includes the article being coated and the atomizedcoating material particles in movement toward such article. In normalcommercial practice it is desirable to mount the article on a groundedconveyor and thus to make the articles themselves (if they areofconducting material) or electrodes within or immediately behind thearticles, at ground potential; and to provide another electrode at ahigh electrical potential with respect to ground, this electrodesometimes being the spraying device and sometimes being a separateelectrode near or past which the spray passes in its movement toward thearticle.

Various means may be used for producing the atomized particles, such aspneumatic spray guns and the like, or the atomizing means may take theform of the extremely efficient spraying device including anelectrostatic atomizing head with a circular atomizing edge of therotary conical type such as disclosed and claimed in the copendingapplication of E. M. Ransburg filed in the United States Patent Officeon February 13, 1950, as Serial No. 143,994.

Where the articles are closely spaced on the conveyor,

the coating zone, which is the zone defined by the electrostatic fieldcreated between theelectrode carrying the high potential and thearticle, follows the nearest article 7 and as such article is moved awayfrom the atomizing means the field, and hence the coating-zone, quitesuddenly transfers to the next succeeding article, resulting in arelatively small change in direction of movement of the particles movingthrough the electrostatic field and hence without substantialinterruption of the coating operation. Where the articles are widelyspacedon the con- 2,744,033 Patented May 1, 1956 may under certaincircumstances be suflicient to permit the leading edge or front portionof the article to pass completely or almost completely out of thecoating zone before the atomized particles can reach it. Undersuchconditions the leading portion of the article is either uncoated or isinsufficiently coated. V

A further difiiculty has arisen in the coating of articles which are soshaped as to'provide certain areas which are more difiicultto coat thanother areas. The difliculty may arise from the particular shape of thearea, such as a flange orthe like, or because one area presents more orless surface to be coated during its passage at a given linear speedthrough the coating zone than preceding or succeeding areas.

One example of an area'which is diflicult to coat, all such areashereinafter being referred to as critical areas, is the leading edge ofa pipe section.

it has been found that when pipe sections are fed to a conveyor which ismoving at a rapid speed of upwards of 200 feet per minute it ispractically impossible consistently to space the sections along theconveyor sulficiently close to each other so that the electrostaticfield transfers itself substantially immediately from the trailing edgeof one pipe section to the leading edge of the following pipe sectionandthus the leading end of the pipe receives insufiicient coating as itpasses the atomizing means although the following noncritical areas ofthe pipe receive an adequate coating. It is believed that the failure ofthe critical area of the pipe to receive adequate coating is due to thefact that while the electrostatic field between the leading end of thepipe and the atomizing head is created substantially instantaneously asthe pipe end nears theshead, the inertia of the atomized particles issuch as to require a certain amount of time beforethey can reach thepipe 'end, during which time such pipe end moves beyond coating range sothat the particles coat other areas of the pipe. The extent of thecritical area is thus further dependent, at least partially, upon theICC speed of movement of the article to be coated and it has been foundthat when coating pipe section, the area at the leading end of the pipewhich is undersprayed may extend backwards therefrom a considerabledistance, which distance is increased as the speed of movement of thepipe is increased.

The particular problem just discussed is often encountered inelectrostatic coating systems in which the particles are atomized atleast in part by electrostatic forces and is not normally found insystems using more conventional atomizing means. Thus, for example,where compressed air is employed to atomize the coating material, theatomized particles are carried by a current of air into theelectrostatic field and toward an article to be coated and the inertiaof the air stream is willcient to carry atomized particles into the pathalong which the article to be coated will be moved. This is true eventhough at a particular instant an article is not veyor, however, thenext following article may be too 7 far removed to act as an electrodeto draw the field to it, and thus there is created a period during whichthe particles are not following a well-defined field and the coatingzone-is substantially nonexistent. When such a situation is coupled withrapid movement of the articles along the conveyor, the next succeedingarticle will, upon its approach, draw the electrostatic fieldinstantaneously to itself. However, a finite time elapses before theparticles may accommodate themselves to the new' field and be movedtherethrough to the article, and such'finite time present in the path.Thus in a system where the atomized particles are carried in a currentor stream-of air, the air may be turned on long prior to the arrival ofthe leading edge of the article at the coating zone and thus, in eifect,a stream of atomized particles may be directed at and through the pathof movement of the article to await the arrival of the leading edge, andthere is no time loss in the movement of particles toward and onto theleading edge. In electrostatic coating systems employing electrostaticatomization, reliance is had upon the electrostatic field existingbetween the atomizing means and the article to direct the particlestoward the article. For this reason unless an article is present and infield creating relationship with the atomizing means the particlesatomized are not directed toward the path 'of movement of the article,but on the contrary are emitted from 7 3 the atomizing means in a ratheraimless pattern. Thus the use of electrostatic atomization ma 'r's'un'isdifficulty in producing coating of proper thickness on the leading edgeof an article moved relatively rapidly along a path adjacent theatomizing means.

The above problems and difliculties can be overcome by the use of themethods and apparatus comprising the present invention and articlespossessing both critical and noncritical areas can be given coatings ofthe desired thickness throughout. The present invention contemplatesvarying the rate of relative movement between the atomizing means andthe article to be coated as the critical area is near the atomizingmeans. The relative speed of movement obviously can either be increasedor decreased depending upon the nature of the critical area as towhether it would otherwise receive an overspray or an underspray. Thusthe relative speed between the article and the atomizing means may beincreased or decreased at the location of the critical area or, asexemplified in the specific embodiment chosen for illustration, theatomizing means itself may be moved, which movement coupled with themovement of the article along the conveyor may e in a direction eitherto increase or to decrease the relative movement therebetween.

Thus the general object of this invention is to provide a method andapparatus for producing a spray coating of predetermined thickness byelectrostatic means upon an article having critical and noncriticalareas. This and other features and objects of the invention will bereadily apparent from the following description and drawings, in which:

Fig. l is a perspective view of one form of coating apparatus embodyingthe invention;

Fig. 2 is a side elevation, somewhat schematic inform, showing a portionof the apparatus; and

Fig. 3 is a view of the upper right-hand portion of Fig. 1 showing amodified form of the invention.

While two dilferent forms of apparatus are illustrated in theaccompanying drawings, and will be described hereafter as providingmeans for practising the present invention, it Will be understood thatthese are representative embodiments only. It will also be understoodthat forms of spraying devices other than the particular electrostaticatomizing devices shown may be utilized, and that other forms ofapparatus may be employed; and it is to be understood that otherembodiments may be utilized without departing from the contemplatedscope of the present invention, and that no limitations are to beimplied from such specific description as shall now be provided.

Referring now to Fig. 1, there is shown one form of apparatus embodyingthe inventions hereof which is illustrated as being employed in thecoating of a plurality of pipe sections and 11 moving along a conveyor12, which carries the pipe sections in a substantially linear path pasta frame 13 which carries the coating apparatus.

The frame includes two pairs of insulated support columns 14- and 15mounted'upo'n suitable bases 16 and 17 with the supports 14 and 1-5carrying at their upper ends plates 18 and 19 respectively. Securedtothe' upper surface of the plates 18an'd 19 is a pair of tracks '20 and21 which are parallel to each other and to the direction of movement ofthe pipe sections along the conveyor. The tracks provide a support for acarriage 22 movable along the tracks on wheels 25 provided thereon.Movement of the carriage along the track is accomplished through themedium of aslot '24 provided in the carriage which is engaged by a pin'25 carried by an endless chain 26 which passes'around a pair ofsprockets 27 and 28 rotatably mountedon angularmembers29'and 30 mountedrespectively on plates Hand 1). Rotation 'of the sprocket 27 to drivethe chain is achieved through a drive shaft 31 which carries at itslower end a bevel gear 32 meshing with a second bevel gear 33'fix'ed toa shaft 34 and rotatably carried by pillow blocks 35" mounted 4 on theplate member 18. The opposite end of the shaft '34 carries a bevel gear3'6 meshing with a bevel gear 37 carried at the upper end of aninsulated drive shaft 38 connected at its lower end by means of a rightangle drive 39 to an electric motor 40.

It can be seen from the foregoing that operation of the motor 40 throughthe shafts and gear means just described drives the chain about thesprockets 27 and 28. Such movement of the chain causes the carriage 22to move back and forth along the tracks 20 and 21 from one end to theother of the'frame 13.

The atomizing means herein may, as previously noted. be of the typeshown in the said copending Ransburg application and includes anelectrostatic atomizing head 41 having an atomizingedge 42 and rotatablymounted in a body portion 43. The atomizing head is suspended from thecarriage by a plurality of support members 43a. Means in the form of aflexible supply line 44 is provided for supplying liquid coatingmaterial to a hollow shaft extending through the body portion andopening at its inner end to an orifice located substantially at thecenter of the atomizing head. Rotation of the head 41 to flow thecoating material outwardly to the atomizing edge is achieved throughgear means which includes a pair of right angle spiral gears 45 and 46.The gear 45 is splined to a shaft 47 rotatably mounted in the frame anddriven, through the gear means 48, insulated drive shaft 49 and rightangle drive 50, by a second electric motor 51.

A-high voltage source 52 is connected by the means 53 to the atomizinghead and by means of connection 54 to a suitableground. Since thesupports 14 and 15 and the shafts 38 and 49 are of electricallyinsulating materials, the entire assembly supported on the plates 18 and19 can be maintained at a high potential with respect to ground.

In the embodiment illustrated in Fig. 1, let it be assumed that the pipesections are carried by the conveyor from left to right past the frame13. As previously pointed out, the critical areas on such pipe sectionsare the leading marginal edge, for example the edge 10a. As the spacebetween the edge 10:: and the trailing edge 11b of the preceding pipe isnormally of such an extent as to prevent the electrostatic field fromthe edge 42 from snapping from the edge 11b to the edge 10a, movement ofthe pipe section 10 rapidly past the head 41 causes underspraying of theedge which in some cases may be accompanied by underspraying of the areaimmediately adjacent the leading edge. To prevent such underspraying,the atomizing head is mounted in the manner just described for movementin a path substantially parallel to the path .of movement of the pipesections and means are provided for initiating movement of the atomizinghead as the critical area, in this case the leading edge 10a, is nearthe'head.

For this purpose there is provided a microswitch 55 adapted to beoperated by a lever arm 56 which is spring urged into the path ofmovement of the pipe section to be contacted by the leading edge 10a.Operation of the'switch 55 through-suitable wiring connection operatesthe motor 40 to move the carriage along the track and hence-to move theatomizing head from a first position at the'left-handend of the frame toa second position at the right-hand end. The motor 40 is designed tomove the atomizing head at approximately the speed of movement of thepipe-section so as to maintain the head and the leading edge of the pipeat approximately their position of closest approach during the periodrequired tomove the head from its first to its second position. When thehead reaches the second position, the carriage contacts the plunger 57of a switch 58 wired to stop the motor uponsuch contact. As theremaining noncritical areas of the pipe pass the atomizing head itremains in its second position to coat such areas. To restart the motor40 to return the head to its first posiswitch 59 is so constructed thatrelease of the arm 60 as the trailing edge of the pipe passes the switchactuates the motor 40 to move the'carriage toward the opposite end ofthe track. As the carriage reaches its initial starting position itcontacts a plunger 61 to operate a switch 62 again to stop the motor.The following pipe section initiates a similar cycle of operations.

It is clear that as the leading edge, such as the edge a, of a pipesection enters the coating 'zone at a high speed of movement relative tothe atomizing head, the atomizing head is moved parallel to and in thesame direction as movement of the edge 10a to permit the atomizedparticles to travel through the electrostatic field to be deposited onthe leading edge. In the particular embodiment of the invention justdescribed, the head moves at substantially the same speed as the leadingedge so that the relative movement is approximately zero. When thecarriage strikes the plunger 57, further motion of the carriage alongthe track is discontinued although the spraying continues along theentire length of the pipe until the trailing edge operates the switch 59to return the head to its first position.

In the modified form of the invention shown in Fig. 3, the switch 59 andits attendant lever arm 60 are omitted and the switch 58 is designed toreduce the speed of the motor 40, for example to one-fourth speed (orsome other fraction or multiple of the speed on its initial movement) sothat the head is returned toward its first position while spraying thenoncritical areaof the article. The speed of the motor upon the returnof the head is so chosen that the remaining areas of the article aresubstantially completely coated before the carriage strikes the plunger61 to halt its motion.

In dealing with articles other than pipe sections which are providedwith critical areas either at their leading edge or in other portions ofthe article, the carriage may be moved at a speed equal to or at speedsother than a speed equal to the speed of movement of the article alongthe conveyor. Thus the atomizing head may be moved, for example atone-half speed, if the critical area is extended in length. Suchmovement of the carriage, and hence of the head, is initiated as thecritical area, wherever it may be located on the article, nears theatomizing head.

While I have shown and described certain embodiments of my invention, itis to be understood that it is capable of many modifications. Changes,therefore, in the construction and arrangement may be made withoutdeparting from the spirit and scope of the invention as disclosed in theappended claims.

I claim:

1. The method of coating an elongated article having a critical areaalong a marginal edge by means of electrostatic deposition of atomizedcoating material particles by an electrostatic field existing betweenthe article and an electrode spaced from the article which comprisesmoving the article, with the marginal edge leading, in a path passingadjacent the electrode, moving the electrode generally parallel to saidpath and in the direction of movement of the article as the criticalarea becomes the terminus of said field to coat said area and thencontinuing the movement of the article without so moving the electrodeto coat immediately following areas of the article.

2. The method of coating an article having a length any times its widthby means of deposition of atomized coating material particles by anelectrostatic field existing between the article and an electrode spacedfrom the article which comprises moving the article longitudinally ofits major axis in a P Passing adiawnt the f"- trode, moving theelectrode generally parallel to said s path in the direction of movementof the article and at approximately the speed thereof as the leading endof vthe article becomes the terminus of said field to coat said end, andthen continuing the movement of the article without so moving theelectrode to coat the remainder of the article.

3. The method of coating a pipe section by means of electrostaticdeposition of electrostaticaily charged atomizing coating materialparticles issued from an atomizing means which comprises moving the pipelongitudinaily of its axis in a path passing adjacent the atomizingmeans, moving the atomizing means generally parallel to said path as theleading end of the pipe nears the atomizing means to maintain theatomizing means at a substantially constant predetermined distance fromsaid end during a portion of the movement of the pipe, and thendiscontinuing such movement of the atomizing means while continuing themovement of the pipe to coat the remainder of the pipe.

4. The method of coating a pipe section by means of electrostaticdeposition of electrostatically charged atomizing coating materialparticles issued from an atomizing means which comprises moving the pipelongitudinally of its axis in a path passing adjacent the atomizingmeans, moving the atomizing means generally parallel to said path from afirst position at approximately the closest approach of the leading endof the pipe to maintain the atomizing means at a substantially constantpredetermined distance from said end during a portion of the movement ofthe pipe, halting the movement of the atomizing means at a secondposition While continuing the movement of the pipe to coat the remainderof the pipe, and then returning the atomizing means to said firstposition.

5. The method of coating a pipe section by means of electrostaticdeposition of electrostatically charged atomizing coating materialparticles issued from an atomizing means which comprises moving the pipelongitudinally of its axis in a path passing adjacent the atomizingmeans, moving the atomizing means generally parallel to said path from afirst position at approximately the closest approach of the leading endof the pipe to maintain the atomizing means at a substantially constantpredetermined distance from said end during a portion of the movement ofthe pipe, halting the movement of the atomizing means at a secondposition, and then returning the atomizing means to said first positionwhile continuing the movement of the pipe to coat the remainder of thepipe.

6. Apparatus for electrostatically coating an article having a lengthmany times its width comprising a conveyor for moving the articlegenerally longitudinally of its major axis in a linear path, atomizingmeans positioned adjacent said path, means for creating an electrostaticcharge difierential between the atomized coating material particles andthe article, a carriage for carrying the atomizing means along a secondpath spaced from and parallel to said linear path, means for moving thecarriage a predetermined distance along said second path, and meansactuated upon the close approach of the leading end of the article tothe atomizing means for operating the carriage moving means to move thecarriage with said end along said second path.

7. Apparatus for electrostatically coating an article having a lengthmany times its width comprising a conveyor for moving the articlegenerally longitudinally of its major axis in a linear path, atomizingmeans positioned adjacent said path, a carriage for carrying theatomizing means along a second path spaced from and parallel to saidlinear path, a motor for moving the carriage a predetermined distancealong said second path, control means positioned along said linear pathopposite one end of the second path and adapted to be operated by theleading end of the article to actuate the motor to move the carriageparallel to said end along said second path, and a second control meansfor reversing the motor to re themarriage to the fi-r'st mentioned endofthe second pa n 8. Apparatus for electrostatically coating a' pipesection comprising a conveyorfor moving the pipe generallylongitudinally of its axis in a linear path, electrostatic atomizingmeans positioned adjacent said path, a carriage for carrying theatomizing means along a second path spaced from and parallel to saidlinear path, an electric motor for moving the carriage a predetermineddistance along said second path, a switch positioned along said linearpath opposite one end of the second path and adapted to be operated bythe leading end of a pipe to energize the motor to move the carriageparallel to said end along said second path, a second switch positionedat the other end of the second path and adapted to be operated by thecarriage as it is moved to said other end to stop the motor, and a thirdswitch positioned along said linear path opposite said other end of thesecond path and adapted to be operated by the trailing end of the pipeto reverse the motor to return the carriage to the first mentioned endof the second path 9. Apparatus for electrostatically coating an articlecomprising a conveyor for moving the article in a linear path, a frameadjacent the path, a track on the frame generally paralleling the path,a carriage movible along the track, atomizing means including a bodyportion and a rotatable atomizing head mounted on the carriage, a splineshaft rotatably mounted in the frame and extending parallel to and inspaced relationship to the track, gear means in said body portionincluding a gear meshing with the spline shaft for rotating the headwith rotation of the shaft, means for rotating the shaft, an electricmotor for moving the carriage along said track, a switch positionedalong said linear path opposite one end of the track and adapted to beoperated by the leading end of an article to energize the motor to movethe carriag'e parallel to said end along said track, and a second switchpositioned at the other end of the track and adapted to be operated bythe carriage as it is moved to said other end to reduce the speed of themotor to return the carriage at a reduced speed to the first mentionedend of the track.

10. Apparatus for electrostatically coating an article comprising aconveyor for moving the article in a linear path, a frame adjacent thepath, a track on the frame generally paralleling the path, a carriagemovable along the track, atomizing means including a body portion and arotatable atomizing head mounted on the carriage, a spline shaftrotatably mounted in the frame and extending parallel to and in spacedrelationship to the track, gear means in said body portion including agear meshing with the spline shaft for rotating the head with rotationof the shaft, means for rotating the shaft, an electric motor for movingthe carriage along said track, a switch positioned along said linearpath opposite one end of the track and adapted to be operated by theleading end of an article to energize the motor to move the carriageparallel to said end along said track, a second switch positioned at the8 otherend ofi thetrack and adapted to be operated by the carriage asismoved to said other end to stop the motor, and a third :switchpositioned along said linear path opposite said other end of'the trackand adapted to be operated by the trailing end of the article to reversethe motor to return the carriage to the first mentioned end of thetrack.

11. In a method of electrostatically coating an article wherein atomizedcoating material is issued at a substantially constant rate from anatomizing means and the atomized particles are deposited by anelectrostatic field maintained by a charging electrode having anelectrical potential constantly applied thereto and wherein the articleis rnoved along a predetermined path having a portion adjacent theatomizing means and the charging electrode, said article having acritical area causing a change in the field strength in the article pathas said area becomes the terminus of said field and wherein such changein field strength causes a change in the concentration of atomizedparticles deposited on such area, the step which comprises automaticallyvarying the rate of relative movement between the article and saidatomizing means directly with said changes in concentration of coatingmaterial particles.

12. The method of claim 11 in which said relative movement is generallyparallel to said path and along with an article moving therealong.

13. The method of claim 11 in which said critical area is the leadingedge of an article and in which said relative movement is generallyparallel to said path as the leading edge of an article moving along thepath approaches the atomizing means to maintain the atomizing meansat asubstantially constant predetermined distance from said edge during aportion of the movement of the article.

14. In a method of electrostatically coating an article wherein atomizedcoating material is issued at a substantially constant rate from anatomizing means and the atomized particles are deposited by anelectrostatic field maintained between an article and the atomizingmeans by constantly applying an electrical potential to said atomizingmeans, and wherein the article is moved along a predetermined pathhaving a portion adjacent the atomizing means, said article having acritical area causing a change in the field strength of said field, andwherein such change in field strength causes a change in theconcentration of atomized particles deposited on such area, the stepwhich comprises automatically varying the rate of relative movementbetween the article and said atomizing means directly with said changesin concentration of coating material particles.

References Cited in the file of this patent UNITED STATES PATENTS2,083,633 Brackett June 15, 1937 2,421,787 Helmuth June 10, 19472,428,991 Ransburg Oct. 14, 1947 2,559,225 Ransburg July 3, 19512,568,611 Crouse Sept. 18, 195l

1. THE METHOD OF COATING AN ELONGATED ARTICLE HAVING A CRITICAL AREA ALONG A MARGINAL EDGE BY MEANS OF ELECTROSTATIC DEPOSITION OF ATOMIZED COATING MATERIAL PARTICLES BY AN ELECTROSTATIC FIELD EXISTING BETWEEN THE ARTICLE AND AN ELECTRODE SPACED FROM THE ARTICLE WHICH COMPRISES MOVING THE ARTICLE, WITH THE MARGINAL EDGE LEADING, IN A PATH PASSING ADJACENT THE ELECTRODE, MOVING THE ELECTRODE GENERALLY PARALLEL TO SAID PATH AND IN THE DIRECTION OF MOVEMENT OF THE ARTICLE AS THE CRITICAL AREA BECOMES THE TERMINUS OF SAID FIELD TO COAT SAID AREA AND THEN CONTINUING THE MOVEMENT OF THE ARTICLE WITHOUT SO MOVING THE ELECTRODE TO COAT IMMEDIATELY FOLLOWING AREAS OF THE ARTICLE. 